1. Field of the Invention
The present invention relates to method and apparatus for processing audio data, and it particularly relates to a technology by which to reduce the noise of the audio data at the time of reproduction thereof.
2. Description of the Related Art
In recent years the coding of digital audio data at high compression ratios has been a subject of intense research and development and the area of its applications is expanding. With the broadened use of portable audio reproducing devices in particular, it is now a general practice that linear PCM signals recorded on, for example, a CD (compact disk) are compressed and recorded on such recording media as small semiconductor memory or minidisk. Also, in modern society where information abounds, data compression technology is indispensable and it is desirable that recording capacity be saved by compressing data to be recorded even on such large-capacity recording media as HD (hard disk), CD-R or DVD. And this compression coding is done by utilizing the most of various technologies including screening of unnecessary signals according to human auditory characteristics, optimization of the assignment of quantized bits, and Huffman coding. Techniques for audio data compression with higher audio quality and higher compression ratios are being studied daily as a most important subject in this field.
In the reproduction of compressed data, the higher the compression ratio is, the greater the quantization error will be, and as a result, there are cases where the reproduced audio data exceeds the original dynamic range of audio data. For example, when 16-bit PCM signals are compressed at a high compression ratio and then decompressed or expanded, there may be instances where expanded data exceeds 16 bits in computation. In such a case, a technique called clipping has conventionally been used, whereby data in excess of 16 bits are substituted into maximum values represented in 16 bits.
At compression ratios required in the conventional practices, there have been few cases where the effect of clipping could be aurally detectable. However, at high compression ratios required today, noises offensive to the ear can often occur as a result of clipping due to the quantization error which is far greater than before. With the compression ratio further rising in the future, this noise problem is expected to grow. Hence, it is believed that clipping by apparatus on the reproduction side only may not suffice to deal with this problem adequately. Described in the following are the experimental data in an analysis of a relationship between clipping and noise.
FIG. 1 shows a relationship between the number of clippings and the presence or absence of noise when audio data are compressed under a fixed compression condition and then expanded and reproduced by a reproduction apparatus. These are the results of an experiment in which 500,000 samples×2 channels were prepared as sound sources. As shown in FIG. 1, sam1 to sam3 are experimental data where audio data from sound sources at high volume were compressed and sam4 and sam5 are experimental data where audio data from sound sources at low volume were compressed. As for the number of clippings, nine consecutive clippings were counted as one count. As is evident in the table, clippings occurred and noise also occurred at reproduction with sam1 to sam3 whereas neither clippings nor noise occurred with sam4 and sam5. This experimental result indicates that under the same compression conditions the higher the volume of sound source, the more likely clippings and noise will occur.
FIG. 2 shows a relationship between the number of clippings and the presence or absence of noise when 500,000 samples×2 channels were prepared as sound sources likely to cause clippings as used with sam1 to sam3 in FIG. 1 and the audio data were compressed under different compression conditions and then expanded and reproduced by a reproduction apparatus. As for the count of clippings, nine consecutive clippings were here counted as one. The frequency bands at compression are those narrowed as a result of compression, indicating that the smaller the value, the higher the compression ratio is. Compression was done in such a way as to remove high-frequency components of data that has been time-frequency converted. For example, the frequency band of 8 kHz of sam6 is to be understood as a frequency band of 0 to 8 kHz after the removal of the high-frequency components above 8 kHz.
The table shows that clippings occurred with all of sam6 to sam10 while noise occurred with sam6 to sam8 but not with sam9 and sam10. Therefore, this experimental result indicates that the occurrence of noise depends on the frequency band secured at compression rather than on the count of clippings.
FIG. 3 shows frequency spectra at reproduction when a sound source of 5 kHz sinusoidal wave is used. The results of this experiment show that there are noise components occurring at 1 kHz and 9 kHz. It is to be noted here that noise components at 15 kHz and above are substantially inaudible to the human ear. It is believed therefore that when there are no audios in the neighborhood of 9 kHz at the reproduction of audio data, the noise component at 9 kHz caused by this 5 kHz sinusoidal wave is detected as a noise offensive to the ear. For example, with sam6 in FIG. 2 wherein compression is done in the frequency band of 0 to 8 kHz, the noise component at 1 kHz may be concealed behind other sounds, but the noise component at 9 kHz can be heard by human ears. The inventors of the present invention consider that one of the reasons for the occurrence of noise as seen in the experimental results of FIG. 2 is the failure to conceal the noise components by other sounds by removing the high-frequency components of the audio data and narrowing the frequency band at compression.